WAEC 2011 Mathematics · Question 26

Simplify log⁡27log⁡81\frac{\log \sqrt{27}}{\log \sqrt{81}}

Options

  1. A.3
  2. B.2
  3. C.32\frac{3}{2}
  4. D.34\frac{3}{4}

Answer: D. 34\frac{3}{4}

Explanation written with AI and checked against the answer key

Analyze the question: We need to simplify a ratio of two logarithms. Note that this is not log(27/81). It is a division of two separate logs, so we cannot use the quotient law directly. Instead, we rewrite each log using powers of 3.

Concept:

  • √x = x^(1/2)
  • log(aⁿ) = n log a

Solution:

Numerator: √27 = (3³)^(1/2) = 3^(3/2) so log √27 = (3/2) log 3

Denominator: √81 = (3⁴)^(1/2) = 3² so log √81 = 2 log 3

Now divide:

log⁡27log⁡81=32log⁡32log⁡3=32×12=34\frac{\log\sqrt{27}}{\log\sqrt{81}} = \frac{\tfrac{3}{2}\log 3}{2\log 3} = \frac{3}{2}\times\frac{1}{2} = \frac{3}{4}

Answer: D. 3/4.

Tip: When you see a ratio of logs of powers of the same base number, convert each to a multiple of the same log (here log 3). The log 3 cancels, leaving a simple fraction. Also remember: log a / log b is NOT log(a/b).

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